Laser-induced near-field patterning of surface at a resolution far below the diffraction limit has attracted more and more attention in recent years due to its extensive potential applications in high-density storage and high-resolution optical lithography for nanodevice fabrication. In most near-field techniques, the subwavelength resolution is achieve by placing a small aperture between the recording medium and light source. If the aperture-to-medium separation is controlled at a distance much smaller than the wavelength, the resolution will be determined by the aperture size instead of the diffraction limit. This technique is used in the scanning near-field optical microscope (SNOM) system; a single hollow optical fiber with a small aperture at its end is used to deliver the laser beam.
Due to the near-field optical enhancement effect at the tip, the SNOM system is able to perform surface modification in a nanoscale of different kinds of materials. However, this approach is difficult to implement in an industrial application due to sophisticated hardware to control the near-field distance and low throughout.
Another approach utilizes a particle mask to pattern a solid substrate. The technique employs a regular two-dimensional (2D) array of microspheres/nanospheres to focus the incident laser radiation onto the substrate. It permits single step surface patterning of thousands or millions submicron holes on the substrate with a single or a few laser shots. The energy conversion efficiency by particle microlens/nanolens is close to 100%, which is significantly higher than that in the SNOM system (10−4-10−5). However, such systems use normal incidence where the laser beam path is perpendicular to the sample surface. The produced nanostructures are formed at positions where spheres were originally located.
The difference between the nanostructures produced by a normal incidence wave and an angular incidence wave is important as that may allow the production of different nanostructures, and to control the nanostructure propositions. Since the intensity of the filed distribution is responsible for the formation of nanostructures, exact examination of field distributions is very important.
The limitation of the majority of previous systems used for determining the intensity of the filed distribution is the use of conventional Mie theory for theoretical modeling, and thus the influence of the substrate on the filed distribution is neglected.